Chapter 7b Fabrication of Solar Cell

Different kind of methods for growth of silicon crystal

Electromagnetic Continuous Casting, not commercially available

Non-wafer Technologies

Specifications of PV Silicon Wafer

Cleaning and texturing

• A layer with thickness of about 10 um has to be etched from both sides of wafers cut by wire saw. The damage removal etch is based on 20~30 wt% aqueous solution of NaOH or KOH heated to 80~90oC.

• The silicon surface after saw damage etching is shiny and reflects more than 35% of incident light.

• Random chemical texturing is used. Monocrystalline silicon substrate (100) can be textured by anisotropic etching at temperature of 70~80oC in a weak, usually 2wt%, solution of NaOH or KOH with addition of isopropanol.

texturing

• This etch produces randomly distributed upside pyramids. However, It has problems of repeatability, lack of pyramid size control, and the presence of untextured regions. The important parameters are: adequate surface preparation, temperature control, mixing rate and isopropanol concentration. This requires the use of appropriate additives which enhance the pyramid nucleation process.

Junction formation, surface passivation and ARC

• Highly doped P in 1020 atms-cm-3

• Typical emitter sheet resistance used in screen printing is between 40 and 60 Ohm/sq.

• Front surface passivation: SiO2

• Antireflection coating: ZnS and MgF2, PECVD nitride (with n in 1.9~2.4)

• Or Coating SiO2 thicker to 110 nm which serves as surface passivation and antireflection coating.

Front contact formation

• Front contact formation: should have a large height-to-width ratio of finger metallization. After contact opening, metallization is obtained by a self-aligned plating process of nickel, copper, and a thin layer of silver.

• In screen printing method, a stainless steel or polyester mesh screen stretched on a metal film frame is covered by a photo-emulsion layer. Openings- which define the front contact pattern- are photolithographically formed in the emulsion layer. Highly conductive silver paste is pushed by a squeegee through the openings in the screen onto substrates with well defined adjustable pressure.

Rear structure

• Alloying a screen printed aluminum paste with silicon. Aluminum can form a eutectic alloy with silicon at a temperature of 577oC. During cooling down, the silicon recrystallizes and is doped with Al at its solubility limit creating a p+ back surface field (BSF) layer. A sufficient thickness of Al is required to achieve a significant contribution of Si in the formation of the liquid phase. Very low back surface recombination velocity (200cm/s) have been reported for thick screen printed and evaporated Al layers (at least 20 μm and 10 μm thick, respectively) fired at a temperature above 800oC.

Gettering – traps metal impurities by phosphorous diffusion

• By phosphorous diffusion, metallic Impurity migration towards gettering sites takes place as a consequence of a large emission of Si interstitials due to the formation of SiP particles by heavy P-diffusion. Enhanced solubility of metallic impurities in such heavily P-diffused regions, and impurity segregation at Si3P4 precipitates lead to efficient gettering. P-diffusion can be performed prior to cell fabrication followed by the removal of the heavily diffused layers (pre-gettering), or a part of emitter formation depending on the optimal gettering conditions required for the material and on the costs involved.

Gettering by aluminum treatment

• Formation of a p+/p high/low junction at the rear side of the cell by re-growth from a fast alloyed Al-Si melt is the most commonly used process for creating the back surface field. This has an additional advantage of bulk gettering by prolonged firing or by a thermal anneal after the initial firing. Fast alloying of screen printed Al-paste by firing, followed by the removal of excess aluminum and a subsequent thermal anneal for up to 1 h, resulted in a considerable enhancement in the bulk diffusion length in large area mc Si wafers.

PV module

EFG: Edge-defined film fed growth, a continuous production of a thin foil or sheet directly from the silicon melt.

MIS: metal oxide semiconductor

High Efficiency Silicon Solar Cell

Cell performance: 18%

Reduce recombination velocity at the front contacts

Micro-groove: 20%

Both contacts are on the rear of the cell: 22%

Efficient light trapping

Screen printing has the problems of large line width and contact resistance

Improved contact resistance by screen printing

Sanyo announced 23% this year for HIT large solar cell

solar cell performance: 20%